474 PROFESSOR GEIKIE ON THE 
lying deep within a volcanic vent, and thoroughly permeated with constantly 
ascending heated vapours, should escape some kind of change. I am inclined 
io attribute to this cause the frequent conversion of the sandstones round 
the walls of the vents into quartz-rock. The most remarkable example 
of metamorphism within the vent itself, which I have observed in the Basin 
of the Firth of Forth, occurs in the great vent of the Campsie Hills.* That 
remarkable volcanic orifice has been filled up with materials, some of which 
present the usual characters of coarse agglomerate, but with a more decidedly 
crystalline matrix. This crystalline texture increases here and there to such a 
degree that the rock assumes the aspect of some of the lavas of the district. 
Yet its original fragmental character is indicated not only by its gradual passage 
into unmistakable agglomerate, but still more by the occurrence in it of 
numerous blocks of sandstone more or less completely converted into quartzite. 
This local and unequal re-crystallization of the volcanic debris of the neck is 
the kind of metamorphism to be looked for as the result of the prolonged ascent 
of superheated steam under some pressure within the pipe of a volcano. I 
may add, that in this same neck numerous veins of a yellow or pink felsitic rock 
may be seen traversing the agglomerate, and extending also into the surround- 
ing bedded porphyrite-lavas. The frequent highly silicated composition of the 
veins in the vents of the porphyrite regions is a remarkable and not very 
intelligible fact. 
2. Intrusive Sheets and Dykes. 
Throughout the Basin of the Firth of Forth, every division of the Carboni- 
ferous system has been invaded by intrusive sheets or dykes of crystalline 
igneous rocks. These masses may sometimes have been connected with the 
surface by vents or cracks up which a portion of the molten material rose. In 
most cases, however, they ought probably to be regarded as hypogene mani- 
festations of volcanic action,—portions of lava which, unable to reach the 
surface, were forced between the bedding, joints, and faults of the strata. It 
will be shown in a later part of this Essay that they possess crystalline 
characters which serve to distinguish them from the superficial lava-streams or 
interbedded sheets. 
General Characters.—The petrography of these rocks will be more particularly 
discussed in the second part of this paper. They consist almost entirely of rocks 
to which the names diabase, dolerite, and basalt may be applied. Occasionally 
they are of a felsitic nature. They include all the more crystalline and granitoid 
rocks of the region, though occasionally they present the ordinary close-grained 
black aspect of basalt. Their texture may be observed to bear some relation to 
* See Explanation to Sheet 31, ‘ Geological Survey of Scotland,” par. 21 (1878), mapped by Mr 
KR. L. Jack, 
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